1. Field of the Invention
The present invention relates to a solid-state laser apparatus and, more particularly, to a high power solid-state laser resonator which is capable of emitting a high-power linearly polarized laser beam.
2. Description of Related Art
In order to output a linearly polarized laser beam from a solid-state laser resonator such as a YAG laser resonator, for example, it is a typical practice to insert an optical device represented by such as a Brewster plate which characterizes a direction of polarization thereof. According to this method, however, it is difficult to output a high-power linearly polarized laser beam due to a birefringence caused by the thermal stress occurring in the laser rod.
As a method for solving such a problem, an arrangement including a quartz polarization rotator which is capable of rotating the polarizing direction by 90 degrees and which is disposed between two laser rods is disclosed in (1) APPLIED PHYSICS LETTERS (Vol. 18, No. 1, pp.3-4, January 1971). According to this scheme, the influence effected from the birefringence induced in one laser rod is canceled out by the induced birefringence associated with the other laser rod. To this end, it is prerequisite that a laser beam be such as to traverse substantially the same portions in both laser rods, and that substantially the same thermal loadings to be applied to both the laser rods.
According to the experiments conducted by the inventors, however, it was found very difficult in practice to successfully arrange equipment for these two prerequisite conditions to be satisfied concurrently. Thus it was found impossible to fully cancel out the influences caused by the effect of the birefringence. As a result, therefore with an increasing thermal loading applied to the laser rod, the output of a linearly polarized laser beam decreased thereby resulting in a non-uniform intensity distribution thereof.
Further, another arrangement for solving the problem is proposed in (2) U.S. Pat. No. 4,935,932 in which a polarization plate and a quarter-wave plate are disposed at both ends of a laser rod, respectively, whereby the polarizer and the quarter-wave plate are disposed such that an incident light beam impinging on the laser rod becomes elliptically polarized or, more preferably, to become circularly polarized.
In the arrangement of (2), it was found that the output of a linearly polarized laser beam decreased with an increasing thermal loading of the laser rod. This phenomenon is presumed to be due to a so-called thermal loading birefringence effect induced in the resonator which allows for the laser beam generated within the resonator to have a component polarized in a different direction from the polarized direction determined by a Brewster plate or the like.
When a birefringence phenomenon takes place, a linearly polarized laser beam having a cross-shaped intensity distribution as shown in accompanying FIG. 15, referred to as the cross mode, is emitted from the laser resonator. An arrow P in FIG. 15 designates the directions of polarization to be characterized by a Brewster inserted therein.
Portions designated by symbol Br in FIG. 15 are regions where a laser beam, subjected to the birefringence phenomenon, has an orthogonal component relative to the direction of polarization to be characterized by a Brewster plate which becomes dominant. Thereby, the overall efficiency of the linearly polarized laser beam is degraded, thus resulting in a significantly reduced output power.